1. Field of the Invention
An optical range simulator device.
2. Description of the Prior Art
The passive optical range simulator described in application Ser. No. 775,061 uses a fiber optic delay line to furnish a single target echo capable of confirming the range calibration and boresight accuracy of an optical radar ranging system. Range calibration of the simulator may be accomplished by any of the following methods: measurement of fiber delay line length and conversion of this to effective optical length using known refractive index of fiber; use of an optical radar ranging system (known to be good) to transfer calibration from surveyed targets on an outdoor range to the simulator being calibrated; or, timing optical pulses propagating on a one-way path through the fiber optical delay line to determine its effective length.
In practice, none of these methods is simple or straightforward, and most simulators have been calibrated by the second method. A target echo from a single known range is sufficient to set or confirm the zero-range adjustment of an optical ranging system under test, provided that the system's range scaling is accurate and behaves linearly. Under these conditions, even though the system has been calibrated only at a single range point, the calibration will be valid at other ranges. In many cases, there is a lack of confidence in system range linearity even though range scale factor may have been set initially by independent electrical measurements.
From the foregoing, it will be apparent that it is desirable to simulate several target echoes separated in range so as to cover the range window of interest. A minimum of three such target echoes is required to distinguish between the effects of zero-set and range-scaling adjustments, and to assess the fundamental ability of a system to time the ranging interval accurately. Thus, zero-range of the system under test may be set or confirmed on any one target echo, range-scaling adjusted (if necessary) to exhibit correct range readout on a second, and proper range readout independently confirmed by means of a third target echo.
There also is a need for multiple target echoes to exercise a ranging system's dynamic response to a multiplicity of targets and change in range of a most distant target.
It is desirable that the multiple target simulator substantially confirm its own calibration in order to avoid the high cost in terms of time and inconvenience associated with the three "independent" calibration techniques identified above. In furnishing the above desired properties, it is important that the ability to confirm boresight accuracy be retained.